This invention relates generally to a tandem type gear pump which can be effectively used in various fields of use of hydraulic machines and instruments.
The tandem type gear pump generally comprises a plurality of pump units so combined as to be driven by a common drive and is useful in case different operating pressures are used simultaneously, in case one after another operating pressure is used, or in case two or more kinds of operating fluid are used.
Japanese Unexamined Utility Model Publication No. 57-8390 or 57-10493 discloses a tandem type gear pump which comprises a housing formed with bores shaped like a pair of glasses, in which three sets of bearing members are axially arranged with a pair of front and rear gear pump units interposed between each adjacent two of the bearing members. Each of the front and rear gear pump units has a drive shaft with a drive gear fixed thereto and a driven shaft with a driven gear fixed thereto and meshing with the drive gear. The drive shafts of the two pump units are connected by a coupling within the intermediate bearing member. When the drive shafts are rotated simultaneously by a drive, each of the pump units individually draws in hydraulic fluid through its inlet port and discharges the fluid through its outlet port.
In the prior art arrangement, the housing in which the bores shaped like a pair of glasses are formed is a body separate from the body in which the bearing bores are formed, so that the positioning of the bores in the separate bodies must be adjusted in separate machining processes. As a result, the relative positions of the bores are likely to be misaligned due to errors in machining or assembly, and gaps are formed between the bores and the outer peripheries of the gears thereby to reduce the volumetric efficiency of the pump. Since the housing and the bearings are separate individual objects, hydraulic fluid is likely to leak out through the gaps therebetween thereby to reduce the volumetric efficiency of the pump. Moreover, if vibration of the bearings acts as an alternating load, fretting corrosion is likely to occur in the bores to increase leakage of hydraulic fluid.
Accordingly, the primary object of the invention is to solve the above problems encountered in the prior art and provide a tandem type gear pump in which the number of machining and assembling steps can be reduced.